1. Field of the Invention
This invention relates to a so-called dual-converter switching power supply having e.g. a power factor improvement circuit and a main switching circuit therein, and more particularly to a switching power supply including a start-up circuit for controlling start-up of two control circuits that control switching operation of the dual converter circuits, respectively.
2. Description of the Related Art
As a dual-converter switching power supply of this kind, there has been conventionally used a power supply 60 shown in FIG. 3 which is a schematic block diagram providing a conceptual representation of the power supply 60. The power supply 60 is comprised of a diode stack 61 for full-wave rectification of an AC current output from an AC power source 2, a power factor improvement converter circuit 62 for boosting a pulsating voltage (voltage of a pulsating current) VD generated by the diode stack 61 and thereby generating a DC voltage V.sub.DC, and a main switching circuit 64 for switching the DC voltage V.sub.DC via a primary winding 63a of a transformer 63. Further, the power supply 60 includes a control IC 65 for controlling switching operation of the power factor improvement converter circuit 62, a capacitor circuit 66 formed e.g. by a capacitor, for supplying an operating current to the control IC 65, a start-up circuit 67 for controlling start-up of the control IC 65, a control IC 68 for controlling switching operation of the main switching circuit 64, a capacitor circuit 69 formed e.g. by a capacitor, for supplying operating current to the control IC 68, and a start-up circuit 70 for controlling start-up of the control IC 68. Moreover, the power supply 60 has a diode 81 arranged on the side of a secondary winding 63b of the transformer 63, for rectifying a voltage induced in the secondary winding 63b and a capacitor 82 also arranged on the secondary winding side, for smoothing the rectified pulsating current.
In the power supply 60, when the power is turned on, the capacitor circuit 66 is charged by a current caused to flow therein via a current path, not shown, by the pulsating voltage V.sub.D. Subsequently, when a charging voltage of the capacitor circuit 66 has reached a predetermined voltage, the start-up circuit 67 starts up the control IC 65. This causes the control IC 65 to start controlling the switching operation of the power factor improvement converter circuit 62, whereby the converter circuit 62 boosts the pulsating voltage V.sub.D to thereby generate the DC voltage V.sub.DC. On the other hand, the capacitor circuit 69 is also charged with a current caused to flow therein via a current path, not shown, by the DC voltage V.sub.DC and when the charging voltage of the capacitor circuit 69 has reached a predetermined voltage, the start-up circuit 70 starts up the control IC 68. This causes the control IC 68 to start controlling the switching operation of the main switching circuit 64, whereby the main switching circuit 64 switches the DC voltage V.sub.DC via a primary winding 63a of the transformer 63. As a result, a voltage is induced in the secondary winding 63b of the transformer 63. The induced voltage is rectified by the diode 81, and the rectified DC voltage is smoothed by the capacitor 82, whereby an output voltage V.sub.O is generated.
As described above, according to this power supply 60, first at the initial stage of power-on, the start-up circuit 67 starts up the control IC 65, and then the start-up circuit 70 starts up the main switching circuit 64, whereby the power factor improvement converter circuit 62 and the main switching circuit 64 are continuously operated for continuous generation of the output voltage V.sub.O.
However, the power supply 60 suffers from the following problems: Firstly, in the power supply 60, the start-up circuit 67 for controlling start-up of the control IC 65 and the start-up circuit 70 for controlling start-up of the control IC 68 are arranged separately and independently of each other, which results in increased manufacturing costs and an increased size of the power supply 60 due to an increase in the number of component parts thereof. Secondly, since the start-up circuits 67 and 70 control the respective control ICs 65 and 68 separately and independently of each other, it is difficult to ensure a reliable start-up sequence of the two control ICs 65 and 68. As a result, the DC voltage V.sub.DC generated by the power factor improvement converter circuit 62 can be switched by the main switching circuit 64 before its voltage value is stable, which can cause a failure in start-up of the main switching circuit 64.